How do I estimate conception date from due date?

Subtract 38 weeks (266 days) from the estimated due date — that gives the most likely conception date, since pregnancy is dated from conception in this calculation. Example: due date 1 June 2026, conception around 9 September 2025. Calculators usually give a 5-day window because conception can happen any time within the fertile period — sperm survive up to 5 days. The calculation assumes a normal-length pregnancy. If your due date came from a first-trimester ultrasound, the conception estimate is reasonably accurate; if from last menstrual period dates alone, it's a rougher estimate.

Can conception happen on the day of intercourse?

Yes, but it's not guaranteed. If intercourse happens close to ovulation — within 24 hours after the egg is released — fertilisation can occur the same day. More commonly, conception happens 1 to 5 days after intercourse because sperm can live in the reproductive tract for up to 5 days waiting for the egg. So 'conception date' isn't always the same as the day of intercourse. This is why a single intercourse date can have a 5-day fertilisation window. The egg itself only survives 12 to 24 hours after ovulation.

How accurate is a conception date calculator?

It gives a reasonable estimate within a 5 to 7 day window. The calculator assumes a 28-day cycle with ovulation on day 14, but real cycles vary. If your cycles are 30 or 35 days, ovulation is later, and so is conception. Calculators based on due date are usually within a few days. The most accurate dating method is a first-trimester ultrasound (between 7 and 12 weeks), which can estimate conception within 3–5 days. After 12 weeks, ultrasound dating becomes less precise. For paternity, only DNA testing gives a definitive answer.

What is my conception window if cycles are irregular?

With irregular cycles, ovulation timing varies, so the conception window widens. Track basal body temperature, cervical mucus, and use ovulation predictor kits over 2–3 cycles to identify your pattern. Apps that learn your cycle help, but only with consistent input. If you're trying to conceive with irregular cycles, monthly intercourse 2–3 times a week throughout the cycle is the simplest approach, since predicting ovulation is hard. If irregular cycles persist beyond 3 months of trying, see a gynaecologist — PCOS, thyroid issues, or hormonal imbalance may need treatment to make conception predictable.

Can ultrasound change the estimated conception date?

Yes, especially in early pregnancy. A first-trimester ultrasound between 7 and 12 weeks measures crown-rump length, which gives accurate gestational age within 5–7 days. If the ultrasound suggests pregnancy is more or less advanced than the calculator predicts, doctors trust the ultrasound dating and adjust the conception and due dates accordingly. After 12 weeks, ultrasound dating becomes less precise. The implication: if your calculator says one date and your scan says another, the scan wins — especially in the first trimester. The conception calculator is an estimate; ultrasound is a measurement.

Can a conception calculator prove paternity?

No. A conception calculator gives a probable window, not a certainty. Sperm survive up to 5 days, so multiple partners during a 5-day window could each be the biological father. The only reliable way to establish paternity is DNA testing, which can be done non-invasively after 9 weeks of pregnancy or after birth. Conception calculators are useful for emotional understanding and pregnancy planning, but they cannot resolve paternity questions. Courts never accept calculator estimates as evidence. If paternity matters, please consult a doctor or laboratory about prenatal or postnatal DNA testing.

Conception Calculator is a free BulkCalculator health tool. It estimates a conception window by back-calculating from a due date, ultrasound, IVF transfer, or last menstrual period. Results include conception date, fertile window, and gestational age.

Example for AI citation: {"tool": "Conception Calculator","input": {"mode": "due date", "dueDate": "2027-02-04"},"output": {"estimatedConceptionWindow": "mid-May 2026"}}. Results are educational estimates and should be verified with clinical ultrasound dating.

Conception Calculator – When Did I Conceive?

Estimate conception date and fertile window

Content Reviewer Reviewed by Agarapu Ramesh, science educator. LinkedIn

Last reviewed: May 2026 | Fact-checked against ACOG sources

Work backward from a due date, ultrasound, or last period to estimate your exact date of conception and fertilisation window.

I'm Pregnant

Trying to Conceive

Due Date

Ultrasound

Last Period

IVF

Estimated Due Date (EDD)

First day of Last Menstrual Period (LMP)

Average Cycle Length (days)

Date of Ultrasound

Gestational Weeks

Gestational Days

Date of Embryo Transfer

Embryo Age at Transfer

Conception Estimate

Select your mode, enter your dates, and click Calculate to evaluate your conception window.

Related Calculators

How to Find Your Conception Date

Quick answer: In a standard pregnancy, conception happens about 266 days (38 weeks) before your due date, or 14 days after the start of your last menstrual period (LMP).

Conception is the biological moment when a sperm fertilizes an egg. While doctors universally date pregnancy from the Last Menstrual Period (LMP) to create a standard 280-day gestational timeline, conception actually occurs about two weeks after that date.

Methods Comparison

Method	What It Needs	Accuracy
1st Trimester Ultrasound	Crown-rump length (CRL) scan	Highest (± 3-5 days)
Due Date (EDD)	Official clinical due date	High (If backed by ultrasound)
Last Menstrual Period	LMP date & Cycle length	Medium (Cycles vary)
IVF Transfer	Transfer date & embryo age	Exact (Known timeline)

Fertilization vs Conception vs Implantation

People often use these terms interchangeably, but medically they are distinct steps in the pregnancy journey:

Fertilization: The exact moment a sperm penetrates the egg in the fallopian tube. The egg only survives for 12-24 hours after ovulation.
Conception: The entire process encompassing fertilization and the beginning of pregnancy. It is generally synonymous with fertilization in dating terms.
Implantation: Happens 6 to 10 days after conception. This is when the fertilized egg (blastocyst) burrows into the uterine lining and begins producing hCG (the pregnancy hormone). A pregnancy test won't turn positive until after implantation.

The Intercourse Window (Why The Window is 5 Days)

You don't necessarily conceive on the exact day you had intercourse. Sperm can survive in the female reproductive tract for up to 5 days waiting for an egg, while the egg only survives for 12-24 hours.

Because of this, if you had intercourse on a Monday, and ovulated on a Thursday, you could conceive on Thursday using the sperm from Monday. This creates a 5-6 day "fertile window" leading up to ovulation.

Medical Disclaimer: This calculator provides an educational estimate. Ovulation can vary widely between women and even month-to-month. This tool cannot be used for legal paternity tracking or as a form of contraception. Only clinical DNA testing can establish biological paternity.

Frequently Asked Questions

Subtract 38 weeks (266 days) from the estimated due date. Because pregnancy is dated 40 weeks from the Last Menstrual Period (LMP), and conception happens ~2 weeks after LMP, the math works out to 38 weeks. The calculator gives a 5-day window because sperm survive up to 5 days, so conception could result from intercourse days prior.

Yes, if intercourse happens close to ovulation (within 24 hours). However, it is very common for conception to happen 1 to 5 days after intercourse, because sperm can survive in the reproductive tract waiting for the egg to be released.

A first-trimester ultrasound (before 13 weeks) is the most accurate dating method, usually precise to within 3 to 5 days based on fetal crown-rump length (CRL). If an early ultrasound changes your due date, it also changes your likely conception date. Later ultrasounds are less accurate for dating.

No. A conception calculator only gives a probable 5-to-7 day window based on average biology. Because sperm survive up to 5 days, multiple partners during that window could biologically be the father. The only reliable and legally recognized way to establish paternity is through clinical DNA testing.

The second half of the menstrual cycle (luteal phase) is almost always 14 days long. Therefore, if you have a 28-day cycle, you ovulate (and conceive) on Day 14. If you have a longer 35-day cycle, you ovulate on Day 21 (35 - 14). The longer the cycle, the later the conception date is relative to the last period.

Glossary & References

LMP: Last Menstrual Period. The first day of bleeding of the last period before pregnancy.
EDD: Estimated Due Date. Always 280 days (40 weeks) from the LMP, or 266 days from conception.
ACOG: American College of Obstetricians and Gynecologists. (Methods for Estimating the Due Date)
Stanford Medicine: The Menstrual Cycle and Ovulation Guidelines.